I've used Prestone green coolant in my 300E and it works really good but a mix of green and yellow/red will induce immediate rusting of engine. Look at the Motorweek website. If they billed you for MB antifreeze then go back and demand a citric acid wash + a complete flush. If it is just pure green, don't worry that much but a mix will ruin the engine. Prestone is safe for all engines. Perhaps MB has released a secret silent bulletin on 300E and green coolant.
Distilled water is very important. Their might be small particulates in your tap water that could destroy your engine. Remember there is no coolant filter or passthrough. pH should be 7.00. If not distilled must be potable water. Air bubbles should be burped out. Ratio should be minumum to protect against freezing. In So. Cal in my 300E I run 25%. Forget about water wetter, I remember a guy on this board who ran Redline and he ended up switching out his headgaskets.